This invention relates to electronic ignition circuits particularly those used in motor vehicles, and particularly to control circuits for preventing excessive coil current.
In a vehicle equipped with an electronic ignition circuit the current supplied to the ignition coil depends to a large measure upon the voltage of the vehicle battery. The battery voltage is subject to considerable fluctuation both as a result of temperature variations and as a result of the condition of battery charge. In order to provide sufficient current through the ignition coil when battery voltage is in a low state, the coil circuit must be designed with sufficiently low resistance to provide the minimum current necessary to effect proper ignition. When the vehicle battery is fully charged, and the ambient temperature is high, the battery will deliver a larger than necessary current to the ignition coil on account of higher battery voltage. The higher current through the ignition coil may result in overheating of the coil with possible coil damage.
Published German patent application No. 2,303,087 discloses a circuit arrangement for controlling current supplied to the ignition coil. This prior arrangement makes use of a circuit which is responsive to the current through the ignition coil, which is measured by the use of a resistance in series with the coil. This prior art circuit enables a considerable reduction in coil current variations, but cannot provide substantially complete compensation since the control circuit is activated by the coil current itself.
It is therefore an object of the present invention to provide a new and improved control circuit for controlling the current through an ignition coil.
It is a further object of the present invention to provide such a control circuit which is responsive to both coil current and battery voltage.